1. Field of the Invention
The present invention relates to exhaust gas purification techniques.
2. Description of the Related Art
Exhaust gas-purifying catalysts for treating exhaust gas emitted from automobiles and the like conventionally contain a precious metal such as a platinum-group element (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 1-242149, Jpn. Pat. Appln. KOKOKU Publication No. 6-75675, and Jpn. Pat. Appln. KOKAI Publication Nos. 10-202101, 2004-041866, 2004-041867, and 2004-041868). The precious metal plays a role as a catalytically active component which promotes a nitrogen oxide (NOx) reduction reaction, and a hydrocarbon (HC) and carbon monoxide (CO) oxidation reaction, and the like.
However, precious metals are expensive and rare, and therefore, the composition of exhaust gas-purifying catalysts should preferably be free of precious metals. Yet, it has been considered very difficult to achieve sufficient exhaust gas purification performance with no precious metal or a small amount of a precious metal.
Exhaust gas-purifying catalysts are often used at high temperature under such conditions that the oxygen concentration changes relatively frequently. Therefore, when exhaust gas-purifying catalysts contain precious metals, the catalytic activity can easily be decreased by grain growth of the precious metals. Accordingly, improving the durability has become a significant challenge to exhaust gas-purifying catalysts containing precious metals.
To suppress the decrease in catalytic activity, for example, there has been developed an exhaust gas-purifying catalyst in which palladium forms a solid solution in a perovskite-type composite oxide (see, for example, Jpn. Pat. Appln. KOKAI Publication Nos. 2004-041866, 2004-041867, and 2004-041868). This catalyst allows palladium in the composite oxide to repeat precipitation and solid solution formation in response to the change in the oxygen concentration of exhaust gas, thereby suppressing grain growth of palladium.
However, the perovskite-type composite oxide is relatively low in heat resistance. Therefore, when used at high temperature for a long time, such a catalyst may suffer reduced self-regenerating capability, which is achieved by the precipitation and solid solution formation of palladium. In some cases, therefore, such a catalyst cannot achieve high exhaust gas purification performance after an endurance test.